Method and device for uplink transmission

ABSTRACT

A method for transmitting data in a wireless local area network and the device using the same are provided. The method includes steps to transmit a request frame from an access point (AP) to a plurality of stations. The stations respond by transmitting physical layer protocol data units (PPDUs) in response to the request frame. The request frame and the responsive PPDU&#39;s are uniquely structured to increase communication efficiency.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of a currentlypending U.S. application Ser. No. 15/671,635 having a U.S. filing dateof Aug. 8, 2017. The U.S. application Ser. No. 15/671,635, in turn, is aContinuation Application of a U.S. application Ser. No. 15/390,619having a U.S. filing date of Dec. 26, 2016. The U.S. application Ser.No. 15/390,619 is a Continuation Application (Bypass ContinuationApplication) of international application PCT/IB2015/001825 having aninternational filing date of 26 Jun. 2015 and designating the UnitedStates, the international application claiming priority to earlier filedKorean patent application 10-2014-0080173 filed on Jun. 27, 2014. Theentire contents of all of these prior applications are incorporatedherein by reference. The applicant claims the benefit of and claimspriory herein to all of these applications and their filing dates andpriority dates.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method for transmitting data in a wireless local areanetwork and a device using the same.

Related Art

Institute of Electrical and Electronics Engineers (IEEE) 802.11nstandard established in 2009 provides a transfer rate of up to 600 Mbpsat a frequency band of 2.4 GHz or 5 GHz on the basis of Multiple InputMultiple Output (MIMO) technique.

IEEE 802.11ac standard established in 2013 aims to provide a throughputgreater than or equal to 1 Gbps utilizing Medium Access Control (MAC)Service Access Point (SAP) layer scheme at a frequency band less than orequal to 6 GHz. A system supporting IEEE 802.11ac standard is referredto as a Very High Throughput (VHT) system.

There are continuing efforts to implement more effective Wireless LocalArea Network (WLAN) technologies in increasingly congested environments.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting data in awireless local area network. The present invention also provides adevice for transmitting data in a wireless local area network.

In one aspect of the present invention, a method for communicating in alocal area network is provided. The method includes a step oftransmitting, at an access point (AP), a request frame to a plurality ofstations, the request frame transmitted at a first instant. The requestframe requests the plurality of stations to transmit a physical layerprotocol data units (PPDUs) in response to the request frame, therequest frame including a clear channel assessment (CCA) signal, the CCAsignal instructing each station to perform CCA before transmitting itsresponsive PPDU. The method includes a second step of receiving, at theAP, the plurality of responsive PPDUs from the plurality of stations,each responsive PPDU received over sub-channels for which CCA wereperformed within a predetermined period of time following the firstinstant. Each responsive PPDU is generated based on an access category(AC) selected by its transmitting station.

For the plurality of responsive PDUS, a transmission opportunity (TXOP)is restricted by a TXOP limit of a primary AC that is used to gainaccess to at least one sub-channel. When any one of the CCA testedsub-channels is not idle, a responsive PPDU is not transmitted. Eachsub-channel may have a bandwidth of 20 MHz. The request frame includesinformation indicating at least one sub-channel over which CCA is to beperformed. The predetermined period of time is at least equal to aninter frame space (IFS).

In another aspect of the present invention, a device used forcommunicating in a local area is provided. The device includes aprocessor and memory disposed to the processor. The memory includesinstructions that, when executed by the processor, causes the device totransmit a request frame to a plurality of stations, the request frametransmitted at a first instant. The request frame requests the pluralityof stations to transmit a physical layer protocol data units (PPDUs) inresponse to the request frame, the request frame including a clearchannel assessment (CCA) signal, the CCA signal instructing each stationto perform CCA before transmitting its responsive PPDU. The memoryincludes instructions that, when executed by the processor, causes thedevice to receive the plurality of responsive PPDUs from the pluralityof stations. Each responsive PPDU are received over sub-channels forwhich CCA were performed by a corresponding station within apredetermined period of time following the first instant. Eachresponsive PPDU is generated based on an access category (AC) selectedby its transmitting station.

For the plurality of responsive PDUS, a transmission opportunity (TXOP)is restricted by a TXOP limit of a primary AC that is used to gainaccess to at least one sub-channel. When any one of the CCA testedsub-channels is not idle, a responsive PPDU is not transmitted. Eachsub-channel may have a bandwidth of 20 MHz. The request frame includesinformation indicating at least one sub-channel over which CCA is to beperformed. The predetermined period of time is at least equal to aninter frame space (IFS).

Using these inventive methods and devices, a greater amount of data canbe transmitted during a same time period; thus, transmission efficiencyis increased. In addition, a Peak-to-Average Power Ratio (PAPR) of atransmitter can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows PPDU formats used by the legacy system.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

FIG. 3 shows constellation phases for the conventional PPDU.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

FIG. 5 shows an HEW PPDU format in a 20 MHz channel.

FIG. 6 shows an HEW PPDU format in a 40 MHz channel.

FIG. 7 shows an HEW PPDU format in an 80 MHz channel.

FIG. 8 shows a PPDU format according to another embodiment of thepresent invention.

FIG. 9 shows bandwidth signaling according to an embodiment of thepresent invention.

FIG. 10 shows an example of PPDU transmission having an RTS/CTSbandwidth signal.

FIG. 11 shows a scrambling procedure for a data field in a PPDU.

FIG. 12 shows an example of HEW PPDU transmission having an RTS/CTSbandwidth signal.

FIG. 13 shows a PIFS Recovery procedure performed after a frame erroroccurs in the middle of TXOP.

FIG. 14 shows a Recovery procedure when a frame error occurs.

FIG. 15 is a block diagram of an STA according to an embodiment of thepresent invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The proposed wireless local area network (WLAN) system may operate at aband less than or equal to 6 GHz or at a band of 60 GHz. The operatingband less than or equal to 6 GHz may include at least one of 2.4 GHz and5 GHz.

For clarity, a system complying with the Institute of Electrical andElectronics Engineers (IEEE) 802.11 a/g standard is referred to as anon-High Throughput (non-HT) system, a system complying with the IEEE802.11n standard is referred to as a High Throughput (HT) system, and asystem complying with IEEE 802.11ac standard is referred to as a VeryHigh Throughput (VHT) system. In comparison thereto, a WLAN systemcomplying with the proposed method is referred to as a High EfficiencyWLAN (HEW) system. A WLAN system supporting systems used before the HEWsystem is released is referred to as a legacy system. The HEW system mayinclude an HEW Station (STA) and an HEW Access Point (AP). The term HEWis only for the purpose of distinguishing from the conventional WLAN,and there is no restriction thereon. The HEW system may support IEEE802.11/a/g/n/ac by providing backward compatibility in addition to theproposed method.

Hereinafter, unless a function of a station (STA) is additionallydistinguished from a function of an Access Point (AP), the STA mayinclude a non-AP STA and/or the AP. When it is described as an STA-to-APcommunication, the STA may be expressed as the non-AP STA, and maycorrespond to communication between the non-AP STA and the AP. When itis described as STA-to-STA communication or when a function of the AP isnot additionally required, the STA may be the non-AP STA or the AP.

A Physical layer Protocol Data unit (PPDU) is a data unit for datatransmission.

FIG. 1 shows PPDU formats used by the legacy system.

A non-HT PPDU supporting IEEE 802.11a/g includes a Legacy-Short TrainingField (L-STF), a Legacy-Long Training Field (L-LTF), and a Legacy-Signal(L-SIG).

An HT PPDU supporting IEEE 802.11n includes a HT-SIG, a HT-STF, and aHT-LTF after the L-SIG.

A VHT PPDU supporting IEEE 802.11ac includes a VHT-SIG-A, a VHT-STF, aVHT-LTF, and a VHT-SIG-B after the L-SIG.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

An L-STF may be used for frame detection, Automatic Gain Control (AGC),diversity detection, and coarse frequency/time synchronization.

An L-LTF may be used for fine frequency/time synchronization and channelestimation.

An L-SIG may include information indicating a total length of acorresponding PPDU (or information indicating a transmission time of aphysical layer protocol service unit (PSDU)).

The L-STF, the L-LTF and the L-SIG may be identical to L-STF, L-LTF andL-SIG of the VHT system. The L-STF, the L-LTF and the L-SIG may bereferred to as a legacy portion. The L-STF, the L-LTF, and the L-SIG maybe transmitted in at least one Orthogonal Frequency DivisionMultiplexing (OFDM) symbol generated on the basis of 64-points FastFourier Transform (FFT) (or 64 subcarriers) in each 20 MHz channel. For20 MHz transmission, the legacy portion may be generated by performingan inverse Discrete Fourier Transform (IDFT) with 64 FFT points. For 40MHz transmission, the legacy portion may be generated by performing anIDFT with 128 FFT points. For 80 MHz transmission, the legacy portionmay be generated by performing an IDFT with 512 FFT points.

A HEW-SIGA may include common control information commonly received byan STA which receives a PPDU. The HEW-SIGA may be transmitted in 2 OFDMsymbols or 3 OFDM symbols.

The following table exemplifies information included in the HEW-SIGA. Afield name or the number of bits is for exemplary purposes only.

TABLE 1 Field Bits Description Bandwidth 2 Set to 0 for 20 MHz, 1 for 40MHz, 2 for 80 MHz, 3 for 160 MHz and 80 + 80 MHz mode STBC 1 Set to 1 ifall streams use STBC, otherwise set to 0. When STBC bit is 1, an oddnumber of space time streams per user is not allowed. Group ID 6 Set tothe value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a HEW SU PPDU; otherwise, indicates a HEW MU PPDU.Nsts/Partial AID 12 For MU: 3 bits/user with maximum of 4 users Set to 0for 0 space time streams Set to 1 for 1 space time stream Set to 2 for 2space time streams Set to 3 for 3 space time streams Set to 4 for 4space time streams Otherwise: first 3 bits contain stream allocation forSU, set to 0 for 1 space time stream, set to 1 for 2 space time streams,etcetera up to 8 streams. Remaining 9 bits contain partial associationidentifier (AID). No TXOP PS 1 Set to 1 to indicate that TXOP PS is notallowed. Set to 0 to indicate that TXOP PS is allowed. Set to the samevalue in all PPDUs in downlink MU TXOP. GI (Guard 2 Set B0 to 0 for LongGI, set to 1 for Short GI. Set B1 to 1 when interval) Short GI. Coding 2For SU: Set B2 to 0 for BCC, set to 1 for LDPC For MU: Set B2 to 0 forBCC, set to 1 for LDPC for 1st user If user 1 has 0 Nsts value, then B2is reserved and set to 1 MCS 4 For SU/Broadcast/Multicast: Modulationand coding scheme (MCS) index For MU: B1: Set to 0 for BCC, 1 for LDPCfor the 2nd user B2: Set to 0 for BCC, 1 for LDPC for the 3rd user B3:Set to 0 for BCC, 1 for LDPC for the 4th user If user 2, 3, or 4 has 0Nsts value, then corresponding bit is reserved and set to 1 SU- 1 Set to1 when packet is a SU-beamformed packet Beamformed Set to 0 otherwiseFor MU: Reserved, set to 1 CRC 8 Tail 6 All zeros

A HEW-STF may be used to improve an AGC estimation in an MIMOtransmission.

A HEW-LTF may be used to estimate a MIMO channel. The HEW-LTF may startat the same point of time and may end at the same point of time acrossall users.

A HEW-SIGB may include user-specific information required for each STAto receive its PSDU. For example, the HEW-SIGB may include informationregarding a length of a corresponding PSDU and/or a bandwidth or channelin which the PSDU for a corresponding receiver is transmitted.

A data portion may include at least one PSDU. The position of theHEW-SIGB is illustration purpose only. The HEW-SIGB may be followed bythe data portion. The HEW-SIGB may be followed by the HEW-STF or theHEW-LTF.

In the proposed PPDU format, the number of OFDM subcarriers may beincreased per unit frequency. The number of OFDM subcarriers mayincrease K-times by increasing FFT size. K may be 2, 4, or 8. Thisincrease may be accomplished via downclocking (e.g., using a larger FFTsize with a same sampling rate).

For example, K=4 downclocking is assumed. As for the legacy portion, 64FFT is used in a 20 MHz channel, 128 FFT is used in a 40 MHz channel,and 256 FFT is used in an 80 MHz channel. As for a HEW portion using thelarger FFT size, 256 FFT is used in a 20 MHz channel, 512 FFT is used ina 40 MHz channel, and 1024 FFT is used in an 80 MHz channel. TheHEW-SIGA may have same FFT size as the legacy portion. The HEW portionmay have larger FFT size than the legacy portion.

The PPDU is generated by performing IDFT with two different FFT sizes.The PPDU may include a first part with a first FFT size and a secondpart with a second FFT size. The first part may include at least one ofthe L-STF, the L-LTF, the L-SIG and the HEW-SIGA. The second part mayinclude at least one of the HEW-STF, the HEW-LTF and the data portion.The HEW-SIGB may be included in the first part or in the second part.

When an FFT size is increased, an OFDM subcarrier spacing is decreasedand thus the number of OFDM subcarriers per unit frequency is increased,but an OFDM symbol duration is increased. A guard interval (GI) (or alsoreferred to as a Cyclic Prefix (CP) length) of the OFDM symbol time canbe decreased when the FFT size is increased.

If the number of OFDM subcarriers per unit frequency is increased, alegacy STA supporting the conventional IEEE 80.2.11a/g/n/ac cannotdecode a corresponding PPDU. In order for the legacy STA and an HEW STAto co-exist, L-STF, L-LTF, and L-SIG are transmitted through 64 FFT in a20 MHz channel so that the legacy STA can receive the L-STF, the L-LTF,and the L-SIG. For example, the L-SIG is transmitted in a single OFDMsymbol, a symbol time of the single OFDM symbol is 4 micro seconds (us),and the GI is 0.8 us.

Although the HEW-SIGA includes information required to decode an HEWPPDU by the HEW STA, the HEW-SIGA may be transmitted through 64 FFT inan 20 MHz channel so that it can be received by both of the legacy STAand the HEW STA. This is to allow the HEW STA to receive not only theHEW PPDU but also the conventional non-HT/HT/VHT PPDU.

FIG. 3 shows constellation phases for the conventional PPDU.

To identify a format of a PPDU, a phase of a constellation for two OFDMsymbols transmitted after L-STF, L-LTF, and L-SIG is used.

A ‘first OFDM symbol’ is an OFDM symbol first appeared after the L-SIG.A ‘second OFDM symbol’ is an OFDM symbol subsequent to the first OFDMsymbol. In a non-HT PPDU, the same phase of the constellation is used inthe 1st OFDM symbol and the 2nd OFDM symbol. Binary Phase Shift Keying(BPSK) is used in both of the 1st OFMD symbol and the 2nd OFDM symbol.

In an HT PPDU, although the same phase of the constellation is used inthe 1st OFDM symbol and the 2nd OFDM symbol, the constellation rotatesby 90 degrees in a counterclockwise direction with respect to the phaseused in the non-HT PPDU. A modulation scheme having a constellationwhich rotates by 90 degrees is called Quadrature Binary Phase ShiftKeying (QBPSK).

In a VHT PPDU, a constellation of the first OFDM symbol is identical tothat of the non-HT PPDU, whereas a constellation of the second OFDMsymbol is identical to that of the HT PPDU. The constellation of secondOFDM symbol rotates 90 degrees in a counterclockwise direction withrespect to the 1st OFDM symbol. The first OFDM symbol uses BPSKmodulation, and the 2nd OFDM symbol uses QBPSK modulation. SinceVHT-SIG-A is transmitted after L-SIG and the VHT-SIG-A is transmitted intwo OFDM symbols, the first OFDM symbol and the second OFDM symbol areused to transmit the VHT-SIG-A.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

To distinguish from a non-HT/HT/VHT PPDU, a constellation of at leastone OFDM symbol transmitted after L-SIG can be used.

Just like the non-HT PPDU, a first OFDM symbol and a second OFDM symbolof the HEW PPDU have the same constellation phase. A BPSK modulation maybe used for the first OFDM symbol and the second OFDM symbol. The STAcan differentiate the HEW PPDU and HT/VHT PPDUs.

In an embodiment, to differentiate the HEW PPDU and the non-HT PPDU, theconstellation of a third OFDM symbol can be utilized. The constellationof the third OFDM symbol may rotate by 90 degrees in a counterclockwisedirection with respect to the second OFDM symbol. The first and secondOFDM symbols may use BPSK modulation, but the third OFDM symbol may useQBPSK modulation.

In another embodiment, the HEW-SIGA may provide an indication about theformat of the PPDU. The indication may indicate whether the format ofthe PPDU is a HEW PPDU. The HEW-SIGA may provide an indication about ause of orthogonal frequency division multiple access (OFDMA).

Hereinafter, a PPDU using a phase rotation in frequency domain isproposed in order to support lower Peak-to-Average Power Ratio (PAPR).

For clarity, it is assumed that the second part (i.e. HEW part) of thePPDU uses 4-times FFT size via downclocking.

Hereinafter, a subchannel refers to a resource allocation unit to beallocated to a STA. Operating bandwidth (i.e. 20 MHz channel, 40 MHzchannel, 80 MHz channel or 160 MHz channel) can be divided into aplurality of subchannels. A subchannel may include one or moresubcarriers. The plurality of subchannels may have same number ofsubcarriers or different number of subcarriers. One or more subchannelscan be allocated to the STA. The STA can transmit one or more PPDUsthrough the allocated subchannels. The subchannel may be referred to as‘a subband’ or ‘a subgroup’.

FIG. 5 shows an HEW PPDU format using 256 FFT in a 20 MHz channel.

The first part (i.e. L-LTF, L-LTF, L-SIG and HEW-SIGA) uses 64 FFT inthe 20 MHz channel. In order to implement the 256 FFT in the secondpart, it is proposed to decrease an overhead by performing ¼down-clocking on a VHT 80 MHz PPDU format and by decreasing GI to 0.8 usand 0.4 us.

If the VHT 80 MHz PPDU format is subjected to ¼ down-clocking, an OFDMsymbol time is increased by four times, and thus is 16 us when usingLong GI, and is 14.4 us when using Short GI. That is, the GI is alsoincreased to 3.2 us in case of Long GI and to 1.6 us in case of ShortGI. However, the GI may keep to 0.8 us in case of Long GI and to 0.4 usin case of Short GI. In doing so, after performing the ¼ downclocking,the OFDM symbol time is 13.6 us when using Long GI and is 13.2 us whenusing Short GI.

If the VHT 80 MHz PPDU format is subjected to ¼ down-clocking in the 20MHz channel, each of 64 FFT-based VHT-STF, VHT-LTF, and VHT-SIG-B mayconstitute one subchannel, and as a result, 4 subchannels are combinedand transmitted through the 20 MHz channel in unit of 256 FFT.

In FIG. 5, in order to decrease a Peak-to-Average Power Ratio (PAPR) ofa transmitter STA, the second part may be subjected to multiplicationfor a phase waveform in unit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},\ {k \geq {{- 6}4}}} \\{{+ 1},\ {k < {{- 6}4}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Herein, R(k) denotes a multiplication value for a phase waveform at asubcarrier index k. 256 subcarriers are divided into 4 subchannels.Respective subchannel is composed of 64 subcarriers. A sequence {+1, −1,−1, −1} may be multiplied for the 4 subchannels, starting from asubchannel having a smallest subcarrier index, that is, a lowermostsubchannel.

The equation 1 can be expressed as follows. The 256 subcarriers aredivided into first and second subgroups that have different number ofsubcarriers. The first subgroup is phase-rotated by multiplying +1 andthe second subgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF may be as follows.

HEW-STF={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58)},

HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft, 1,LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright}

where:

-   -   HTS_(−58,58)=√{square root over (½)} {0, 1, 1+j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0,        0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0,        0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0},

LTFleft={1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1, 1,−1, 1, −1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}.

FIG. 6 shows an HEW PPDU format in a 40 MHz channel.

In order to implement the 512 FFT in the 40 MHz channel, it is proposedto use two blocks for the aforementioned 256 FFT transmission of the 20MHz channel. Like in the 256 FFT transmission in the 20 MHz channel, anOFDM symbol time is 13.6 us when using Long GI, and is 13.2 us whenusing Short GI.

L-STF, L-LTF, L-SIG, and HEW-SIGA are generated using 64 FFT and aretransmitted in a duplicated manner two times in the 40 MHz channel. Thatis, the first part is transmitted in a first 20 MHz subchannel and itsduplication is transmitted in a second 20 MHz subchannel.

In order to decrease a PAPR of a transmitter STA for transmitting theL-STF, the L-LTF, the L-SIG, and the HEW-SIGA, multiplication may beperformed on a phase waveform in unit of 20 MHz channel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{+ j},\ {k \geq 0}} \\{{+ 1},\ {k < 0}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

This means that the first part is phase-rotated by multiplying +1 forthe first 20 MHz subchannel and is phase-rotated by multiplying +j forthe second 20 MHz subchannel.

The equation 2 can be expressed as follows. The 128 subcarriers aredivided into first and second subgroups. The first subgroup isphase-rotated by multiplying +1 and the second subgroup is phase-rotatedby multiplying +j.

For each 64 FFT-based subchannel constituting 512 FFT, in order todecrease a PAPR of a transmitter STA for transmitting HEW-STF, HEW-LTF,and HEW-SIGB, multiplication may be performed on a phase waveform inunit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},{{64} \leq k}} \\{{+ 1},{0 \leq k < {64}}} \\{{- 1},\ {{{- 1}92} \leq k < 0}} \\{{+ 1},\ {k < {{- 1}92}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

More specifically, according to Equation 3, 512 subcarriers are dividedinto 8 subchannels. Respective subchannel is composed of 64 subcarriers.A sequence {+1, −1, −1, −1, +1, −1, −1, −1} may be multiplied for the 8subchannels, starting from a subchannel having a smallest subcarrierindex, that is, a lowermost subchannel.

The equation 3 can be expressed as follows. The 512 subcarriers aredivided into four subgroups. The first subgroup is phase-rotated bymultiplying +1, the second subgroup is phase-rotated by multiplying −1,the third subgroup is phase-rotated by multiplying +1, and the fourthsubgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF may be as follows.

HEW-STF={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, HTS_(−58,58)},

HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft, 1,LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft, 1, LTFright, −1, −1, −1, 1, 1,−1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1,−1, 1, LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright}

Herein,

-   -   HTS_(−58,58)=√{square root over (½)} {0, 1, 1+j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0,        0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0,        0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0},

LTFleft={1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1, 1,−1, 1, −1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}.

FIG. 7 shows an HEW PPDU format in an 80 MHz channel.

In order to implement the 1024 FFT in the 80 MHz channel, it is proposedto use four blocks for the aforementioned 256 FFT transmission of the 20MHz channel. Like in the 256 FFT transmission in the 20 MHz channel, anOFDM symbol time is 13.6 us when using Long GI, and is 13.2 us whenusing Short GI.

L-STF, L-LTF, L-SIG, and HEW-SIGA which are transmitted using 64 FFT arealso transmitted in a duplicated manner four times in the 80 MHzchannel. That is, the first part is transmitted in a first 20 MHzsubchannel and its duplications are transmitted in second, third andfourth 20 MHz subchannels respectively.

In order to decrease a PAPR of a transmitter STA for transmitting theL-STF, the L-LTF, the L-SIG, and the HEW-SIGA, multiplication may beperformed on a phase waveform in unit of 20 MHz channel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},\ {k \geq {{- 6}4}}} \\{{+ 1},\ {k < {{- 6}4}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

This means that the first part is phase-rotated by multiplying +1 forthe first 20 MHz subchannel and is phase-rotated by multiplying −1 forthe second, third and fourth 20 MHz subchannels.

The equation 4 can be expressed as follows. The 256 subcarriers aredivided into first and second subgroups that have different number ofsubcarriers. The first subgroup is phase-rotated by multiplying +1 andthe second subgroup is phase-rotated by multiplying −1.

For each 64 FFT-based subchannel constituting 1024 FFT, in order todecrease a PAPR of a transmitter STA for transmitting HEW-STF, HEW-LTF,and HEW-SIGB, multiplication may be performed on a phase waveform inunit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},{{256} \leq k}} \\{{+ 1},{{192} \leq k < {256}}} \\{{- 1},{{64} \leq k < {192}}} \\{{+ 1},{0 \leq {k64}}} \\{{- 1},\ {{{- 1}92} \leq k < 0}} \\{{+ 1},{{256} \leq k \leq {{- 1}92}}} \\{{- 1},\ {{{- 4}48} \leq k < {{- 2}56}}} \\{{+ 1},\ {k < {{- 4}48}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

More specifically, according to Equation 5, 1024 subcarriers are dividedinto 16 subchannels. Respective subchannel is composed of 64subcarriers. A sequence {+1, −1, −1, −1, +1, −1, −1, −1, +1, −1, −1, −1,+1, −1, −1, −1} may be multiplied for the 16 subchannels, starting froma subchannel having a smallest subcarrier index, that is, a lowermostsubchannel.

The equation 5 can be expressed as follows. The 1024 subcarriers aredivided into 8 subgroups. The first subgroup is phase-rotated bymultiplying +1, the second subgroup is phase-rotated by multiplying −1,the third subgroup is phase-rotated by multiplying +1, the fourthsubgroup is phase-rotated by multiplying −1, the fifth subgroup isphase-rotated by multiplying +1, the sixth subgroup is phase-rotated bymultiplying −1, the seventh subgroup is phase-rotated by multiplying +1and the eighth subgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF is as follows.

-   -   HEW-STF={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,        HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0,        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0,        0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,        HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0,        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58)},

HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft, 1,LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft, 1, LTFright, −1, −1, −1, 1, 1,−1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1,−1, 1, LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft, 1,LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft, 1, LTFright, −1, −1, −1,1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, 0, LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft, 1,LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright},

Herein,

-   -   HTS_(−58,58)=√{square root over (½)} {0, 1, 1+j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0,        0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,        0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0,        0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j,        0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0},

LTFleft={1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1, 1,−1, 1, −1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}

An FFT size can be increased to improve PPDU transmission efficiency. Inorder to provide compatibility with the legacy STA, the first part (STF,LTF, L-SIG and HEW-SIGA) using the same FFT size as the legacy PPDU isfirst transmitted, and subsequently the second part (HEW-STF, HEW-LTF,HEW-SIGB and a PSDU) using a larger FFT size are transmitted.

In order to decrease a PAPR of a transmitter STA, the first part and thesecond part uses different phase rotation in frequency domain. It meansthat a phase rotation for subcarriers in the first part is differentfrom a phase rotation for subcarriers in the second part.

FIG. 8 shows a PPDU format according to another embodiment of thepresent invention.

Since the number of OFDM subcarriers per unit frequency increases aftertransmitting L-STF, L-LTF, L-SIG, and HEW-SIGA, a processing time may berequired to process data with larger FFT size. The processing time maybe called an HEW transition gap.

In an embodiment, the HEW transition gap may be implemented by defininga Short Inter-Frame Space (SIFS) followed by the HEW-STF. The SIFS maybe positioned between the HEW-SIGA and the HEW-STF. The SIFS may bepositioned between the HEW-SIGB and the HEW-STF.

In another embodiment, the HEW transition gap may be implemented in sucha manner that the HEW-STF is transmitted one more time. The duration ofthe HEW-STF may vary depending on the processing time or STA'scapability. If the processing time is required, the duration of theHEW-STF may become double.

Hereinafter, a proposed bandwidth signaling is described.

A transmitter STA may transmit a Request To Send (RTS) frame to adestination STA before transmitting an HEW PPDU. Further, thetransmitter STA may receive a Clear To Send (CTS) frame from thedestination STA as a response. A transmission bandwidth of the HEW PPDUmay be determined using a bandwidth signal through RTS/CTS exchangebetween the transmitter STA and the destination STA.

If the transmitter STA performs Clear Channel Assessment (CCA) and if itis determined that a 40 MHz channel is idle, the RTS frame istransmitted through the 40 MHz channel. The destination STA receives theRTS frame only in the 20 MHz channel if only the 20 MHz channel is idle,and the destination STA responds with the CTS frame to the transmitterSTA in the 20 MHz channel. Since the transmitter STA transmits the RTSframe through the 40 MHz channel but receives the CTS frame as aresponse only in the 20 MHz channel, a transmission bandwidth of an HEWPPDU may be less than or equal to a channel bandwidth in which aresponse is received using the CTS frame.

FIG. 9 shows bandwidth signaling according to an embodiment of thepresent invention. An STA1 is a transmitter STA, and an STA2 is adestination STA.

Before transmitting an HEW PPDU, the STA1 transmits an RTS frame to theSTA2, and receives a CTS frame from the STA2. The STA1 performs CCA, andsince it is determined that an 80 MHz channel is idle, transmits the RTSframe through the 80 MHz channel in unit of 20 MHz channel in aduplicated manner. That is, four 20 MHz RTS frames (i.e., one 20 MHz RTSframe and three duplicated RTS frames) are transmitted at an 80 MHzband. For the purpose of decreasing a PAPR of an STA for transmittingthe RTS frame, a value of {1, −1, −1, −1} may be multiplied each 20 MHzchannel.

In the STA2, only a 40 MHz channel is idle and thus the RTS frame isreceived only through the 40 MHz channel. The STA2 responds with the CTSframe to the STA1 in the 40 MHz channel.

Although the STA1 transmits the RTS frame through the 80 MHz channel,the CTS frame is received only through the 40 MHz channel. Therefore, atransmission bandwidth of an HEW PPDU transmitted at a later time may beset to a 40 MHz channel bandwidth at which a response is received usingthe CTS frame.

The CTS frame may also be transmitted in a duplicated manner in unit of20 MHz. For the purpose of decreasing a PAPR of the STA2 fortransmitting a plurality of CTS frames, a value of {1, j} may bemultiplied each 20 MHz channel.

The HEW PPDU can be transmitted simultaneously to a plurality ofdestination STAs by a transmitter STA by independently dividing achannel. In FIG. 9, as to a PSDU transmitted by the STA1, one PSDU istransmitted to the STA2 by using the lowermost 20 MHz channel, and atthe same time, another PSDU is transmitted to an STA3 by using a 20 MHzchannel thereon. However, optionally, it is also possible that thetransmitter STA, i.e., STA1, performs transmission to only onedestination STA without having to independently divide all availablechannels.

When the HEW PPDU is simultaneously transmitted to a plurality ofdestination STAs by independently dividing a channel, a channelbandwidth of each PSDU addressed to each destination STA may be limitedto be less than or equal to a channel bandwidth in which a response isreceived using the CTS frame. Also, a sum of channel bandwidths of allPSDUs in the HEW PPDU may be limited to be less than or equal to achannel bandwidth in which a response is received using the CTS frame.After exchanging RTS/CTS frame, the HEW PPDU being simultaneouslytransmitted to a plurality of destination STAs may have a PSDU addressedto a STA responding a CTS frame. In FIG. 9, because the STA2 respondswith the CTS frame, the PSDU addressed to the STA2 is included in theHEW PPDU.

A phase rotation sequence of a HEW PPDU may be determined based on atransmission bandwidth of the HEW PPDU. A phase rotation sequence of aHEW PPDU transmitted to either a single destination STA or transmittedto a plurality of destination STAs is same when the transmissionbandwidth of the HEW PPDU is identical. In FIG. 9, an HEW PPDU using 512FFT in a 40 MHz channel is applying the same phase rotation sequence asdescribed in FIG. 6 although the PSDUs of HEW PPDU are addressed to aplurality of destination STAs.

Now, a method related to a PPDU transmission and an error recoveryduring a transmission opportunity (TXOP) is described.

A TXOP may be defined as an interval of time during which a STA has theright to initiate frame exchange sequences onto a wireless medium. Anaccess category (AC) may be defined as a label for the common set ofenhanced distributed channel access (EDCA) parameters that are used by astation to contend for the channel in order to transmit medium accesscontrol (MAC) service data units (MSDUs) with certain priorities. The ACrelates to quality-of-service (QoS) requirements.

If a STA transmits one or more PPDUs simultaneously to a plurality ofdestination STAs by independently transmitting the PPDUs on eachchannel, this may be called as an OFDMA mode. While operating in theOFDMA mode, the STA can send one or more PPDUs to the plurality ofdestination STAs via plurality of channels as shown in FIGS. 8 and 9.

A subchannel may refer to a transmission unit allocated to eachdestination STA in the OFDMA mode. An operating bandwidth can be dividedinto a plurality of subchannels. If a transmitter STA transmits an eachHEW PPDU simultaneously to a plurality of destination STAs byindependently transmitting the HEW PPDU on each channel, the each HEWPPDU to be transmitted to a specific destination STA has the same AccessCategory. In FIG. 9, a PPDU transmitted from an STA1 to an STA2 and aPPDU transmitted by the STA1 to an STA3 must have the same AccessCategory. A TXOP Limit is set differently depending on an AccessCategory of the TXOP. Therefore, this implies that the same TXOP Limitvalue must be applied to all PPDUs to be transmitted, if the transmitterSTA transmits the HEW PPDU simultaneously to the plurality ofdestination STAs by transmitting the HEW PPDU on each channel. For this,a Primary Access Category is proposed.

The Primary Access Category may indicate an Access Category of aBack-off timer used by an STA2 to acquire a TXOP. In FIG. 9, a Back-offtimer is running for each Access Category before an STA1 transmits anRTS frame, and if a Back-off timer corresponding to Access CategoryVoice (AC_VO) reaches 0 among the Back-off timers, the AC_VO correspondsto the Primary Access Category. If the Primary Access Category isdetermined, the HEW PPDU with the Primary Access Category can only betransmitted.

Since each of the plurality of destination STAs has a different amountof data to be received, HEW PPDUs of different Access Categories can besimultaneously transmitted by independently dividing it for each channelaccording to another embodiment of the present invention. However, inthis case, a TXOP Limit of the corresponding TXOP must be determined bythe Primary Access Category. In FIG. 9, when the Primary Access Categoryis the AC_VO, an Access Category of a PPDU transmitted by the STA1 tothe STA2 must be the AC_VO, and the entire TXOP is restricted by theTXOP Limit of the AC_VO. An Access Category of a PPDU transmitted by theSTA1 to the STA3 may be AC_VI (Video), AC_BE (Best Effort) or AC_BK(Background).

If an available bandwidth of a destination STA is wider than atransmission bandwidth of a transmitter STA which acquires a TXOP, thedestination STA may support simultaneous transmission performed byanother STA by independently dividing it for each channel, in additionto the transmitter STA.

The transmitter STA which has acquired the TXOP through the Back-offmechanism transmits an RTS frame to the destination STA. The bandwidthsignal and Access Category may be included in the RTS frame. On thebasis of the bandwidth and Access Category included in the RTS frame,the destination STA may allow another STA to transmit a data frame forthe destination STA. During the TXOP of the transmitter STA, a channelnot used by the transmitter STA is allowed to be used by another STA. Adestination STA can transmits at least one CTS frames via at least oneidle subchannel. For example, the destination STA may send a first CTSframe via a first subchannel to the transmitter STA and may send asecond CTS frame via a second subchannel to another STA. The transmitterSTA which has received the first CTS frame can transmit a data frame tothe destination STA by utilizing only the first subchannel whichreceives the first CTS frame. The destination STA can also utilize thesecond subchannel to communicate with another STA.

FIG. 10 shows an example of PPDU transmission having an RTS/CTSbandwidth signal.

Before transmitting an HEW PPDU, a transmitter STA, i.e., an STA2,transmits an RTS frame to one destination STA, i.e., an STA1, andreceives a CTS frame as a response from the STA1. The STA2 performsClear Channel Assessment (CCA). The STA2 determines that an 80 MHzchannel is idle, transmits the RTS frame through the 80 MHz channel inunit of 20 MHz channel in a duplicated manner. In order to decrease aPAPR, a phase rotation sequence of {+1, −1, −1, −1} is multiplied overfour 20 MHz channels.

In a case where the destination STA, i.e., the STA1, intends to supportsimultaneous transmission of an HEW PPDU by a plurality of transmitterSTAs by independently dividing it for each channel, a CTS frame may betransmitted to different transmitter STAs for each channel as aresponse. In FIG. 10, it can be seen that the STA1 responds with the CTSframe to the STA2, and at the same time, STA1 sends with a CTS frame toan STA3 in a different channel. Although the CTS frame is simultaneouslytransmitted by being independently divided for each channel with respectto different transmitter STAs, it can be seen that transmission isperformed by multiplying four 20 MHz channels by a phase rotationsequence of {+1, −1, −1, −1}.

The STA2 and STA3 can receive the CTS frames from the STA1 respectively.Respective CTS frame has information about its transmission channel andan Access Category. STA2 and STA3 can send HEW PPDUs to the STA1 viatransmission channels in which corresponding CTS frames are received.

The HEW PPDUs may have same Access Category. In FIG. 10, a HEW PPDU1transmitted by the STA2 to the STA1 and a HEW PPDU2 transmitted by theSTA3 to the STA1 may have the same Access Category. A TXOP Limit is setdifferently depending on an Access Category of the TXOP. Therefore, sameTXOP Limit can be applied to all HEW PPDUs to be transmitted. For this,the aforementioned Primary Access Category may be defined.

The Primary Access Category indicates an Access Category of a Back-offtimer used by a STA to acquire the TXOP. In FIG. 10, a Back-off timer isrunning for each Access Category before an STA1 transmits an RTS frame.If a Back-off timer corresponding to Access Category Voice (AC_VO)reaches 0, the AC_VO corresponds to the Primary Access Category. If thePrimary Access Category is determined, information about the PrimaryAccess Category can be sent to a destination STA. The destination STAcan deliver the Primary Access Category information to the plurality oftransmitter STAs. Accordingly, all PPDUs to be transmitted by theplurality of transmitter STAs can have same Access Category.

Since the plurality of transmitter STAs have a different amount of datato transmit, HEW PPDUs of different Access Categories can besimultaneously transmitted by independently dividing it for each channelaccording to another embodiment of the present invention. However, inthis case, a TXOP Limit of the corresponding TXOP must be determined bythe Primary Access Category. In FIG. 10, when the Primary AccessCategory is the AC_VO, an Access Category of a PPDU transmitted by theSTA1 to the STA2 must be the AC_VO, and the entire TXOP is restricted bythe TXOP Limit of the AC_VO. An Access Category of a PPDU transmitted bythe STA1 to the STA3 may be AC_VI (Video), AC_BE (Best Effort) or AC_BK(Background).

To deliver information about the Primary Access Category to STAs throughan RTS/CTS frame, it is proposed to encode at least one bit of thescrambling sequence with a QoS parameter such as AC_VO, AC_VI, AC_BE,AC_BK.

FIG. 11 shows a scrambling procedure for a data field in a PPDU.

A data field in a PPDU may be scrambled with a length-127frame-synchronous scrambler. The data field includes at least one PDSU.The octets of the PSDU are placed in the transmit serial bit stream, bit0 first and bit 7 last. The 127-bit sequence generated repeatedly by thescrambler shall be (leftmost used first), 00001110 11110010 1100100100000010 00100110 00101110 10110110 00001100 11010100 11100111 1011010000101010 11111010 01010001 10111000 1111111. The same scrambler is usedto scramble transmit data and to descramble receive data. If theparameter CH_BANDWIDTH_IN_NON_HT is not present, the initial state ofthe scrambler may be set to a pseudo-random nonzero state. If theparameter CH_BANDWIDTH_IN_NON_HT is present, the first 7 bits of thescrambling sequence may be set as shown in following table.

TABLE 2 First 7 bits of Scrambling Sequence B0 B1 B2 B3 B4 B5 B6 PrimaryAccess Category z z z CH_BANDWIDTH_IN_NON_HT

Since the first 7 bits of the scrambling sequence are used as ascrambling initial seed, at least 2 bits may be set to a valueindicating the Primary Access Category.

When a Primary Access Category of a corresponding TXOP is known throughan RTS frame, a destination STA can respond with a CTS frame by settingthe Primary Access Category to the same value.

FIG. 12 shows an example of HEW PPDU transmission having an RTS/CTSbandwidth signal.

This is a case where an STA1 responds with a CTS frame to an STA2 and anSTA3, but the STA3 fails to successfully receive the CTS frame. The STA2acquires the TXOP and the STA1 is the destination STA.

If the STA3 fails to successfully receive the CTS frame, the STA3 doesnot transmit a data frame to the STA1. As such, if an error occurs inthe middle of TXOP, the data frame is not transmitted in a channelallocated to the STA3. In order to utilize the channel not used by theSTA3, the STA1 and the STA2 may perform a PCF Interframe Space (PIFS)recovery procedure on all of a primary channel and secondary channels todetermine again a channel bandwidth to be used at a later time.

FIG. 13 shows a PIFS Recovery procedure performed after a frame erroroccurs in the middle of TXOP.

An STA2 acquires TXOP through a Back-off timer of an AC_VO, andsubsequently transmits an RTS frame to an STA1. The STA1 responds with aCTS frame to the STA2 and an STA3 by using different channels. The STA2which has successfully received the CTS frame transmits a PPDU to theSTA1 by using a bandwidth signal included in the CTS frame and a channelthrough which the CTS frame is received. Further, a Block ACK frame isreceived from the STA1 as a response, and a feedback for data frametransmission is received.

However, the STA3 which fails to successfully receive the CTS frame doesnot transmit any PPDU to the STA1.

The STA1 which cannot receive any data frame from the STA3 requests theSTA2, i.e., a TXOP owner, to perform a PIFS Recovery for the purpose ofre-allocating to another STA a channel allocated to the STA3. Such arequest may be signaled through a Block ACK frame transmitted by theSTA1 to the STA2. The STA2 which receives a request for performing thePIFS Recovery from the STA1 may determine whether a channel state is anidle/busy state by performing a CCA process during a PIFS time withrespect to a primary channel and secondary channels.

If the STA1 has a right of the TXOP owner (e.g., the STA1 is a RDresponder in reverse direction protocol), the STA1 may perform the CCAprocess during a PIFS time with respect to a primary channel andsecondary channels. It means that a STA operating in the OFDMA modeperforms the PIFS Recovery for the purpose of re-allocating a channelduring a TXOP, irrespective of the success of the transmitted HEW PPDU.

In FIG. 13, all 80 MHz channels are idle, and the STA2 transmits againan RTS frame in the 80 MHz channel. A destination STA, i.e., the STA1,responds with a CTS frame to the STA2 and the STA3 through respectivedifferent channels, and thus provides the STA3 an opportunity ofsimultaneously transmitting an HEW PPDU independently in a correspondingchannel one more time. In this time, the STA3 which has successfullyreceived the CTS frame also transmits a PPDU to the STA1 by using abandwidth signal included in the CTS frame and a channel through which aCTS frame is received. Further, a Block ACK frame is received from theSTA1 as a response, and a feedback for data frame transmission isreceived.

FIG. 14 shows a Recovery procedure when a frame error occurs.

An STA2 acquires TXOP through a Back-off timer of an AC_VO, andsubsequently transmits RTS frames to an STA1. The STA1 responds with CTSframes to the STA2 and an STA3 by using different channels.

The STA3 which has successfully received the CTS frame transmits a PPDUto the STA1 by using a bandwidth signal included in the CTS frame and achannel through which the CTS frame is received.

However, the STA2 which fails to successfully receive the CTS frame doesnot transmit any PPDU to the STA1. Since the STA2 corresponding to aTXOP owner does not use a primary channel, all STAs including the STA2perform a Back-off mechanism again, and in the above figure, an STA4 cannewly obtain TXOP and transmit RTS frames to the STA1. However, sincethe STA3 is currently transmitting a 40 MHz PPDU, a correspondingchannel state is busy, and thus the RTS frames of the STA4 can betransmitted only through a 40 MHz channel including a primary channel.This is a case where the STA1 receives PPDUs from the STA2 and alsoreceives the RTS frames from the STA4.

In an embodiment, a STA can stop receiving of a frame which is currentlybeing received in secondary channels when a certain frame is received inits primary channel while another frame is received in the secondarychannels. A capture effect is a scheme of immediately stopping receivingof a frame currently being received upon receiving of a signal havingstrength greater by a specific level than or equal to received signalstrength of a frame currently being received in the same channel. Theproposed method extends such a concept of the capture effect, whichmeans that receiving of a certain frame is immediately stop irrespectiveof reception signal strength of a frame currently being received insecondary channels, when the certain frame is received in its primarychannel during the certain frame is received in the secondary channels.

In FIG. 14, the STA1 which has successfully received an RTS frame fromthe STA4 responds with a CTS frame to the STA3, and subsequently, theSTA4 starts to transmit a PPDU to the STA1.

FIG. 15 is a block diagram of an STA according to an embodiment of thepresent invention.

The STA may include a processor 21, a memory 22, and a Radio Frequency(RF) module 23.

The processor 21 implements an operation of the STA according to theembodiment of the present invention. The processor 21 may generate aPPDU according to an embodiment of the present invention and mayinstruct the RF module 23 to transmit the PPDU. The memory 22 storesinstructions for the operation of the processor 21. The storedinstructions may be executed by the processor 21 and may be implementedto perform the aforementioned operation of the STA. The RF module 23transmits and receives a radio signal.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for communicating in a local areanetwork, the method comprising: transmitting, at an access point (AP), arequest frame to a plurality of stations, the request frame transmittedat a first instant, wherein the request frame requests the plurality ofstations to transmit a physical layer protocol data units (PPDUs) inresponse to the request frame, the request frame including a clearchannel assessment (CCA) signal, the CCA signal instructing each stationto perform CCA before transmitting its responsive PPDU; and receiving,at the AP, the plurality of responsive PPDUs from the plurality ofstations, each responsive PPDU received over sub-channels for which CCAwere performed within a predetermined period of time following the firstinstant, wherein each responsive PPDU is generated based on an accesscategory (AC) selected by its transmitting station.
 2. The method ofclaim 1, wherein a transmission opportunity (TXOP) for the plurality ofresponsive PPDUs is restricted by a TXOP limit of a primary AC that isused to gain access to at least one sub-channel.
 3. The method of claim1, wherein a responsive PPDU is not transmitted by any station when anyone of the CCA tested sub-channels is not idle.
 4. The method of claim1, wherein each sub-channel has a bandwidth of 20 MHz.
 5. The method ofclaim 1, wherein the request frame includes information indicating theat least one sub-channel over which CCA is to be performed.
 6. Themethod of claim 1, wherein the predetermined period of time is at leastequal to an inter frame space (IFS).
 7. A device comprising: aprocessor; and memory disposed to said processor, said memory comprisinginstructions that, when executed by said processor, causes the deviceto: transmit a request frame to a plurality of stations, the requestframe transmitted at a first instant, wherein the request frame requeststhe plurality of stations to transmit a physical layer protocol dataunits (PPDUs) in response to the request frame, the request frameincluding a clear channel assessment (CCA) signal, the CCA signalinstructing each station to perform CCA before transmitting itsresponsive PPDU; and receive the plurality of responsive PPDUs from theplurality of stations, each responsive PPDU received over sub-channelsfor which CCA were performed by a corresponding station within apredetermined period of time following the first instant, wherein eachresponsive PPDU is generated based on an access category (AC) selectedby its transmitting station.
 8. The device of claim 7, wherein atransmission opportunity (TXOP) for the plurality of responsive PPDUs isrestricted by a TXOP limit of a primary AC that is used to gain accessto at least one sub-channel.
 9. The device of claim 7, wherein aresponsive PPDU is not transmitted by any station when any one of theCCA tested sub-channels is not idle.
 10. The device of claim 7, whereineach sub-channel has a bandwidth of 20 MHz.
 11. The device of claim 7,wherein the request frame includes information indicating the at leastone sub-channel over which CCA is to be performed.
 12. The device ofclaim 7, wherein the predetermined period of time is at least equal toan inter frame space (IFS).